Steam cracking is used to crack various hydrocarbon gaseous (e.g., light alkanes) or liquid (e.g., naphthas) feedstocks into higher value products, such as olefins, preferably light olefins such as ethylene and propylene. In addition to naphthas, other liquid feedstocks of interest may include, for example, distillation residues or bottoms, gas oils, kerosenes, crudes, various other liquid separation product streams, and blends thereof. When steam cracking liquid feedstocks having final boiling points higher than naphthas, the process often produces undesirable by-products, such as various aromatic compounds, ash, metals, coke, asphaltenes, and other high weight materials including molecules that tend to combine to form high molecular weight materials commonly known as tar. Similarly, cracking heavier liquid feedstocks (e.g., feeds having a final boiling point above 260° C.) generally produces more tar and asphaltenes than lighter liquid feeds such as naphthas. The term “final boiling point above X” means that at temperature X, a sample of the material still exhibits at least some non-volatized portions, at least a further portion of which may still be volatized at a temperature greater than X.
Tar is a high boiling point, viscous, reactive material comprising many complex, ringed and branched molecules that can polymerize and foul equipment under certain conditions. Tar also typically contains high-boiling and/or non-volatile components including paraffin-insoluble compounds, such as pentane-insoluble (PI) compounds or heptane-insoluble (HI) compounds, which are molecules of high molecular weight, multi-ring structures, collectively referred to as asphaltenes. Asphaltene accumulation or build-up can progress for a time under various post-cracking conditions, particularly as the steam cracker effluent cools, especially as the tar-containing effluent cools below 300° C.
Tar and associated asphaltenic materials can precipitate, build up in, and plug piping, vessels, and related equipment downstream of the cracking furnace. Further, asphaltenic materials reduce the economic value and further processability of tar by rendering the tar highly viscous and less compatible for mixing or blending with highly paraffinic streams or for use with fuel streams. When so blended, the paraffinic streams and asphaltenes can further induce precipitation of the paraffin-insoluble components in the resulting mixture. Various methods are known in the art to treat tars, such as those produced from steam cracking liquid feedstocks.
U.S. Pat. No. 2,873,245, incorporated herein by reference in its entirety, discloses hydrogen donor diluent cracking of heavy oil, e.g., vacuum residuum, and treating the resulting gas oils in a catalytic cracker. U.S. Pat. No. 3,691,058, incorporated herein by reference in its entirety, discloses an integrated visbreaking-hydrocracking process to break down steam cracker tars into single-ring aromatics. U.S. Pat. No. 3,707,459, incorporated herein by reference in its entirety, discloses visbreaking residua, e.g., thermal tar from steam cracking, in the presence of free radical acceptors, e.g., CaO, isooctane, and n-heptane. U.S. Pat. No. 4,430,197, incorporated herein by reference in its entirety, discloses treating thermal cracker feed with hydrogen donor solvent, separating and rehydrogenating spent hydrogen donor solvent with recycle to the cracking step, and heat soaking the pitch fraction from the cracked products to reduce pentane insolubles with recycle to the cracking step.
U.S. Pat. No. 4,814,065, incorporated herein by reference in its entirety, discloses accelerating hydrogen exchange between a hydrogen donor and a petroleum resid feed for cracking, visbreaking, or coking, by adding aqueous ammonium sulfide and heat soaking. U.S. Pat. No. 5,215,649, incorporated herein by reference in its entirety, discloses producing gaseous olefins by cracking a hydrocarbon feedstock stream wherein the cracked product stream is quenched to stop cracking, followed by injecting hydrogen donor diluent, e.g., dihydronaphthalenes, which suppress molecular weight growth reactions forming undesirable high molecular weight materials such as asphaltenes. U.S. Pat. No. 6,187,172, incorporated herein by reference in its entirety, discloses treating asphaltene-containing feeds to reduce viscosity by adding an asphaltene dispersant such as tetralin or furan. U.S. Pat. No. 6,190,533, incorporated herein by reference in its entirety, discloses converting hydrocarbons such as visbreaker oil or deasphalted oil into steam cracked products by hydrotreating to remove organic sulfur and/or nitrogen compounds, and then passing to a steam cracking zone.
DE 4308507 discloses reducing viscosity of heavy oil residues by treatment at high temperature (427° C.) with a hydrogen donor solvent comprising a fuel oil from steam cracking, which contains hydroaromatic compounds. U.S. application Ser. No. 12/023,204, filed Jan. 31, 2008, discloses upgrading steam cracker tar by heating from below 300° C. to a temperature above 300° C. for a time sufficient to convert at least a portion of the steam cracker tar to lower boiling molecules. U.S. application Ser. No. 12/099,971, filed Apr. 9, 2008, discloses upgrading steam cracker tar by heating from below 300° C. to a temperature above 300° C. and in the presence of steam for a time sufficient to convert at least a portion of the steam cracker tar to lower boiling molecules and separating the heated steam cracker tar into a tar-lean product and a tar-rich product boiling above the tar-lean product. U.S. application Ser. No. 12/112,704, filed Apr. 30, 2008, discloses a process and apparatus for steam cracking heavy feeds, including steam cracked tars. A steam cracked tar feed is heated to provide a depolymerized steam cracked tar containing lower boiling molecules than the steam cracked tar feed. U.S. application Ser. No. 12/486,813, filed Jun. 18, 2009, discloses upgrading steam cracker tar in the presence of steam for a time sufficient to convert steam cracker tar to lower boiling molecules which reduces yields of tars from steam cracking while increasing yields of higher value products.